The PL81 was designed for universal mains sets without a mains transformer and with the heaters on a series chain. The parameters given are for a pair in push pull audio service.
As a line output valve in television service, the peak anode surge voltage is 7,000 Volts. The anode dissipation is 8 Watts.
The PL81 is also useful as a series pass element in voltage regulators.
The robust construction has ceramic strips firmly attached to both working faces of the anode.
The ceramic strips supporting the anode are present to provide an insulated fixing method due to the high peak voltage found in line output service in raster generation for cathode-ray tubes.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 73 mm tall.
References: Data-sheet & 1040. Type PL81 was first introduced in 1951. See also1951 adverts.
